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NitWit Blog

Av Designers – Why Your Tool Box Should Include Optical Screens

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Here are some real world examples from our Race to Black series. Let’s use a room with an ambient light level of 30fc and the screen with 10fc of light on it. Using a typical projection screen, which has an ambient reflected value of 35%, then the best black level possible would be 10 x .35 = 3.5fc. To get a 20:1 contrast ratio you would have to have a projector that would enable the white point to be 3.5x20 = 70fc. That is achievable, but very bright and uncomfortable to look at. Realistically you would have to have a projector achieve 30fc and no more to be comfortable, and the max contrast would be 30 / 3.5 = 8.57:1. Viewable but poor at best.

Using an optical screen with an ALR value of just 5% the same room would have a black level of 10 x 0.05 = 0.5. So to hit a contrast level of 20:1 the projector would only need to be able to hit a white point of 0.5x20 = 10fc. Now that would be very easily done but the white level should be between 75% and 150% of the room ambient with 1:1 being perfect. So if you were to use a projector that could hit just 30fc on screen then it would be very comfortable and offer an outstanding contrast ratio of 30fc / 0.5fc = 60:1. 

At the end of the day it’s easy to see based on the example above, that an
optical screen can make a huge difference in image quality. The room was barely capable of supporting projection originally, but with an optical screen it was capable of achieving great contrast and image quality. So where prior a good designer would have only used a light emitting technology such as a flat panel or direct view LED, now a projection system could be used. Combine that with the fact that it really is a race to black, optical screens are a must have in every designer’s tool set. 

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Double the Power, Double the Contrast

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Let’s examine the Race to Black theory a bit more starting with Table A Black vs Contrast. This chart shows what happens when the black level is reduced while a white level remains constant. This would be similar to an optical projection screen.  That technology has the ability to reduce ambient light at a different factor than projection light; effectively removing only ambient light and lowering the black level while leaving the projection light. In Table A the projection light level used is 400 nits at all times. It is ONLY the black level changing. The black level starts out at 50 then decreases to 1 by increments of 1. So its change is 50/1 = 50; or stated another way, it is just 1/50 times less light than where the black level started. As it does this, the contrast changes as well.  It starts at a value of 0.16 but then increases to 400. So in total, the contrast change is 400/0.16 = 2500. So the contrast went up 2500 times, when the black level only changed by a factor of 50 times. What this shows is that a change in black level for an image is not proportional to the change contrast.            

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Contrast is KING!

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When it comes to display technologies, projection systems have been around a very long time.  There have been many improvements in the technology but nearly all of them apply to the projector.  There have been many variations in projector imaging systems, such as three gun CRT, LCD, DLP engines; as well as light engine improvements from electron gun to UHP and Xenon bulbs. So as a system, projection grew but the screens stayed similar being some form of coated vinyl, particulate bead or a mix of both.  Around 2004, screen technologies that factored in ambient light rejection started to emerge in the market, and that enabled projection to begin to really progress. This is because optical projection screens can remove ambient light without removing the projection light. Meaning that the projection systems can be used in areas that only light emitting technologies were capable of being used.

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